Molecule Masses and Molecular Sieve Pore Sizes

Awhile back we received a question on one of our blog articles that asked the following:

Can you provide sizes of molecules of Isopropyl Alcohol (IPA), Methylene Chloride (DCM) and Water for comparison with 3A and 4A sieves? Thank you.

We forwarded this to our onsite engineer, got the following answer, and decided to post it as a blog article for anyone who had similar questions about finding the volume of a molecule based off of density and mass.

The answer to this question is not as straight forward as it sounds. There are a couple of different ways to look at it. You have to remember it is not only the size of the molecule but the shape. Isopropyl alcohol is a three dimensional structure while n-propyl alcohol is linear. Linear molecules can go into a zeolite pore “end first” so the only thing presented to the pore is the –OH group or the methyl group at the other end. There is also the “radius of gyration” phenomenon to consider. Isomers are often separated from linear molecules using molecular sieve because the isomers are too big to fit while the linear molecules are not even though they have the same molecular weight and density.

1) The average volume of a molecule based on density and mass.

Critical Properties for calculations

IPA

DCM

H2O

Molecular Mass, g/mol

60.1

84.93

18.02

Density, g/cm

0.786

1.33

1

Volume of a mol of a molecule

IPA

DCM

H2O

Volume: cm3/mol

76.46

63.86

18.02

Avagadros Number

6.02E+23

Size of a molecule, Avagadros number

IPA

DCM

H2O

Volume: cm3/molecule

1.27E-22

1.06E-22

2.99E-23

2) Calculating the size from the size of the atoms and the average bond length. The following was considered for these calculations “presented size”, formation, gyration, atomic radius, and bond length. It’s important to remember that molecules change size and shape depending on temperature, pressure, electrical environment and gyrate or flex continuously. Thus the end results can only be approximations due to these extraneous factors

Species

Approximate Angstrom

IPA

16.05

DCM

9.21

Water

2.63

With a 2.63 approximate angstrom size water can be adsorbed by any standard molecular sieve, while DCM can only fit in the pores of a 13X molecular sieve. IPA can’t fit in any standard molecular sieve pore sizes, but there are specially designed zeolites and molecular sieves that can be made that do.

Molecular sieve are crystalline metal aluminosilicates that belong to the zeolite family. That means that the molecules and atoms that make up a molecular sieve are made out of alumina, silicon, and oxygen and because they are crystalline they have a strong degree of order in the way they are laid out.

Molecular sieves specialize in separating very small molecules and atoms apart from one another. Being part of the zeolite family, molecular sieve has a three dimensional network of pores which can adsorb molecules of a specific size. The pores on a molecular sieve is what makes sieve special, this is because they can separate any substance down to the 1/10,000,000,000th of a meter, or an Angstrom. There are four standard pore sizes that a molecular sieve can have:

3A, 3 Angstrom pore size

4A, 4 Angstrom pore size

5A, 5 Angstrom pore size

13X, 10 Angstrom pore size (depending on the manufacturer the pore size may be either 8 or 9 Angstrom)

The pores on molecular sieve could have one of two structure types: A structure or X structure. 3A, 4A, and 5A are made from an A structure while 13X is made from an X structure. The A structure is smaller and more square-shaped than the X structure which is larger and circle shaped.

Aluminum Hydroxide, Sodium Hydroxide, Sodium Bicarbonate, and clay are used in the sieve manufacturing process, when the process is created this combination of material will make 4A molecular sieve when created with a type A structure or 13X molecular sieve when created with a type X structure.

3A and 5A molecular sieve are made once they are ion exchanged with the originally cre

ated 4A sieve. 4A molecular sieve is ion exchanged with potassium to create 3A sieve, the potassium molecules are larger than the sodium molecules they were exchanged with shrink the pore size. 5A sieve is created when 4A sieve is ion exchanged with calcium, calcium molecules are exchanged in a 1:2 ratio. Every calcium molecule removes two sodium molecules thus increasing the size of the pore.

The various pore sizes of molecular sieve offer a great variety of services to anyone looking to separate different combinations of molecules from one another.

A Guide to Determine the Value of Sieve in Ethanol Dehydration

All molecular sieves are not the same. They are not a commodity and the quality varies from manufacturer to manufacturer, therefore it is important to take the time to examine not only the price of molecular sieve but the value.

Virtually all molecular sieve manufacturers measure the same characteristics and properties in molecular sieve, and it is the various measurements of these characteristics that allow you to determine the value of your sieve.

Although this list focuses on determining the value of ethanol grade sieve a lot of these measurements can help determine the suitability of a sieve product for any particular application. Ultimately knowing what makes sieve valuable can make a difficult buying decision less complicated. Listed below are the sieve properties that can help you determine its value.

Density – Knowing the density (when coupled with water adsorption) allows you to figure out the overall water capacity of a vessel in terms of volume or mass. Higher capacity = more water adsorbed. A more valuable sieve has a higher volumetric capacity.

Particle size and distribution – Allows for the calculation of pressure drop, fluidization parameters, and critical velocity through the bed which ultimately effects flow rate. A higher quality sieve has a tight distribution with less “tails.”

Static water adsorption – This refers to the overall capacity of the sieve to adsorb water. (Do not confuse with working capacity which is much less than static capacity and varies with the operation as well as the sieve). For more information on working capacity see my previous article on calculating working capacity. A sieve with a higher static water adsorption capacity is always better.

CO2 adsorption – This measures how much ethanol is being adsorbed with the water in your dehydration beds. Water and (sometimes ethanol) can be adsorbed by 3A sieve because 3A is made from 4A sieve and as a result the sieve bed will not entirely be made up of 3A. Some of the left over 4A sieve adsorbs CO2 and ethanol therefore the higher the CO2 adsorption rate is the higher the ethanol co-adsorption rate in the bed is. This ultimately reduces the overall working capacity per cycle in an ethanol plant, look for low CO2 adsorption rates.

Crush strength – This one’s simple, the higher the crush strength the higher the durability of the molecular sieve beads in operation. A higher number here means a higher quality sieve.

Attrition – This refers to fryability, which is the tendency of the sieve beads to grind up, which produces dust, thus lowering the overall capacity of the bed. A lower attrition number is better.

Ethanol ΔT (Methanol Delta T) – This is a measurement of the ability of sieve to adsorb ethanol, or a measurement of the co-adsorption characteristics of water and ethanol. If capacity is being taken up by ethanol then the water capacity suffers, which is why a lower number is better.

Feel free to use this list as a guide to determine if the sieve you are currently using or or wish to buy is going to be a quality product. You can find most of, or all of this, information about your sieve by asking your supplier or manufacturer for a certificate of analysis from their quality control department.

What Is It and How Does It Separate Oxygen from Nitrogen

What is carbon molecular sieve?

Carbon molecular sieve is an adsorbent that fuses the ideas behind both activated carbon and zeolites into one product. Activated carbon is known for its high porosity and zeolites are known for their ability to be crafted into highly specialized adsorbents called molecular sieve. Carbon molecular sieve is a product that brings the benefits of both of these products together.

Carbon molecular sieve is made out of coal (the same material most activated carbon is made out of) and it specializes in adsorbing material under 10 angstroms, something activated carbon can not do accurately. The smallest pore size created for carbon molecular sieve is 4A, but it exists in a 5A, and 10A (or 13X) as well.

Carbon molecular sieve specializes in separating oxygen from nitrogen, an important part in natural gas processing. This process is done with a PSA (Pressure Swing Adsorption) device in two phases. The first phase sees the gas enter the PSA generator and the oxygen is adsorbed while the nitrogen passes through because the nitrogen molecules are too large and are used as a separate product. The second phase sees the oxygen slowly released from the sieve at low pressures and thereby regenerating it so that the separation process can be repeated.

Carbon molecular sieve is used in this situation as opposed to activated carbon because the physical size between oxygen (0.28nm×0.40nm) and nitrogen (0.30nm×0.41nm) molecules are so close. The pore sizes on carbon molecular sieve are able to accommodate these small size differences, where as activated carbon would just end up adsorbing both of them.

Molecular sieve isn’t used because it is a polar adsorbent, meaning its surface area attracts other polar molecules. Oxygen is a non-polar molecule and would be attracted to other non polar surfaces. Carbon molecular sieve is one of the few non-polar adsorbents out there which is why it is chosen over molecular sieve for this application.

In addition to separating nitrogen from oxygen carbon molecular sieve can be used for metal heat treatment, electron production, and as preservative in food products.

Molecular Sieve 3A, 4A, 5A, and 13X…

What’s the difference?

So what’s the difference between all of these molecular sieve types? – The difference is the size of the pores that come on each molecular sieve bead. The A in 3A stands for Angstrom,a unit of measurement named after Swedish scientist Anders Jonas Angstrom who was looking for a unit of measurement small enough to measure spectral lines (beams of light). An angstrom is equal 1/10 of a nanometer, or 1/10,000,000,000 of a meter, so when speaking of 3A sieve it refers to the size of the pore on the bead which is 3 angstroms or 3/10,000,000,000 of a meter. (On a side note here 13X equals 10A).

So why would someone chose 3A over 4A? – The answer depends on what you are trying to accomplish with your molecular sieve. For example 3A vs 4A, ethanol producers try to make ethanol that is over 99% pure ethanol. Traditional distillation methods only give them a 95% ethanol purity rate, while the remaining 5% of the substance is mostly water. In short they need to separate the final 5% of the water from the ethanol. Ultimately the choose a 3A molecular sieve here is why.

For this example 3A sieve works best because the size of water molecule is approximately 2.8 angstrom and the size of an ethanol molecule is 3.8 angstrom. The 3A sieve adsorbs all of the water molecules because they are small enough to fit inside the pores. The ethanol molecules, which are too large to fit in the pores, are free to pass by thus separating water from ethanol. If this person were to use 4A, 5A, or 13X sieve it would not work because the pore sizes are large enough to adsorb both the ethanol and water molecules, and thus no separation would occur.

Generally speaking 3A sieve is used for purifying methanol and ethanol. 4A is used for removing C02 and ammonia from natural gas streams as well as being a desiccant for refrigerants, medicines, and electrical components. 5A is used for sweetening natural gas and purifying hydrocarbon gas and liquid streams. 13X (which is really 10 Angstrom) is a multipurpose sieve, it can adsorb the all the particles that previous 3 sieves can adsorb, but it is usually used to sweeten natural gas streams and purify petrochemical liquids and gases. Ultimately the pore size of sieve can have a very specific use like 3A or it can have wide range of uses like 13X, it all depends on what you wish to accomplish.